"Ultralight" or "Featherweight"?

[Bruce Taylor]

Or white cedar lapstrake, for that matter.

As some of you know, I've been designing an open camping dinghy. The boat (LOA 14', beam 4') will ride on a car from time to time, and might sometimes need to be launched from a bridge embankment or hoisted over a beaver dam. So, I'm building as light as I can (understanding, of course, that the craft will need to be treated gently, like an Adirondack guideboat or cedar canoe).

My question is: which of the common methods of construction yields the lightest boat?

Having built a cedar lapstrake canoe, I know these boats can be very light indeed (my Piccolo is a bit over 50 lbs, & Rushton's Sairy Gamp was just a bit heavier than a newborn baby: 10 1/2 pounds).

My first thought was to build an "ultralight" lap/ply boat. Tom Hill seems to enjoy carrying his boats on his fingertips, like a barmaid, and that certainly makes them look light. However, I'm told that his fingers are enormously strong from years of planing plywood against the grain, LOL.

And then there's glassed cedar-strip, as popularized by "Mac" McCarthy, Ted Moores, and others. That certainly seems to make for a lightweight boat.

So: which should I use?

For this project, I'm not really too interested in geodesic aerolite, carbon-fibre layups, or any other space-age miracle substances. I don't feel like making a skin boat, either.

Here's a rough model of the dinghy:

http://www.imagestation.com/picture/sraid36/pe8f62492203124b6f69979af1a06077a/fd256776.jpg

[ 10-22-2002, 12:35 PM: Message edited by: Bruce Taylor ]

[htom]

I'll guess that cold-molded is the lightest "wooden" boat. Probably also the most hours of labor and highest cost, especially if you really try for lightest.

[ACB]

ER hum...my 9ft lap/ply stem dinghy is 90lbs and I know I could have saved 10 lbs of that easily; with a lot more finicking work and a much thinner ply I might have saved another 10lbs. But I don't really think it is a lightweight construction.

Glass is heavy stuff, so I don't favour the cedar strip/ glass approach.

My suggestion is cold moulding; you could get away with two skins of cedar, one of 1/16", diagonal, and one of 1/8", fore and aft, I fancy. No framing needed, and just a light centerline structure. Transom of thin ply, framed out.

[Jack Heinlen]

Bruce,

I don't know which would give the lightest weight. Probably more than one contender, depending on how far you wanted to push it. But I see this boat lapstrake. Glued lap...?

I've always had a soft spot for small lapstrake skiffs. Nothing against the other methods, but the beauty of well lined lapstrake planks would just fit here. Don'cha think?

Jack

[Garrett Lowell]

My vote is lapstrake. But that's a personal preference.

[Wild Wassa]

We dinghy racers have different definitions of ultra and featherweight.

13ft Sabre 50lbs. Fully rigged.
12ft Moth 68lbs. Fully rigged. It is radical picking up a 12ft boat by one's self.

Warren.

[ 10-22-2002, 01:50 PM: Message edited by: Wild Wassa ]

[JimConlin]

If weight is the only criterion, you might build it a little bit lighter in glued okoume lapstrake or cold molded than in cedar strip strip composite. Traditional lapstrake would be heavier than either. The strip composite would be stronger and more scuff resistant. If you're going to subject the boat to some rough usage, then strength and scuff resistance become issues. I'd lean toward cedar strip composite, tailoring the glass layup to the degree of exposure to whacks. Typical weight for a cosine wherry (strip composite, 14' x 4') is about 100 lbs. With care, you might get to 80 lbs. or so.

I'd suggest that you make some test panels, weigh them and beat them up

[Jack Heinlen]

Bruce,

A flash of intuition(and we all know some of those are real stinkers). How 'bout pushing the strip built past the edge, with very thin cedar, and then incorporating whatever Monfort uses for his 'guy wires', kevlar I think, and then covering with the lightest fabric, set in epoxy?

You'd have to take care to cover interior chafe areas well, and the outside might look spider webbed, and maybe I don't understant geodesic aerolite construction well at all.

Could be a whole new technique! You can mention me in your credits. ;)

Jack

[Stephen Hutchins]

How about Balsa core with mahogany veners? Also; The other day I was working with some 7/8 inch thick construction foam that was very flexible with the foil pealed off. I though: why not rip this stuff into three inch strips, and use it as a core material. Probably open cell foam, but that wouldn't stop me from experimenting with it. No sir. Lack of funds would though!

[Scott Rosen]

I think glued plywood lap is the best compromise for your intended use. From the exterior, it will look like traditional lap, if you make the strakes narrow enough. On the inside, you'll have very little framing, so it will be clean looking and easy to paint and keep clean.

Strength is good. Weight is low for the strength. To save even more weight, you could use thicker ply for garboard and lower strakes and a thinner ply for the topsides. It will take a beating better than traditional lap.

Also important, it can tolerate being stored dry and won't leak when you first put it in the water.

I suppose cold-molded has many of the same advantages, but it would require much more epoxy work, and I'm not sure you would save any weight. Plus, you'd lose the lapstrake look.

[NormMessinger]

Forget wood composite. Let's get modern and very light and strong. Use Divinycell foam and 10 oz BID fibreglass. See http://wicks.overcoffee.com/gotopage.php?page=24 . Marine grade would be cheaper if you can find it--Defender maybe. Three plys inside and out below water line, two above. Painted you wont know it's not wood except it will almost float in air.

--Norm

[Bruce Taylor]

Interesting answers. I hadn't given much thought to cold moulding -- partly because it seemed like a lot of work, but mainly, I have to admit, because no enterprising author has coined a catchy phrase calling attention to the lightness of cold moulded boats (as Tom Hill and Mac McCarthy have done for their favorite methods).

Warren, what modern miracle stuff are these high-performance dinghies made of?

Scott -- So far, I'm leaning to your logic.

Norm -- NOW I'm well and truly befuddled. I didn't even know this magic foam existed. Is this what your plane is made of? How do you work it?

[Wild Wassa]

Bruce, to call them high performance, their not (if compaired to a 505 or 49er for example), their real benefit, is that youngsters can launch them.

Standard Marine 3ply, Douglas Fir for the structural components. Not much else. Waterproofed and thinly painted. Verneers tend to be either Coachwood or Australian Red Cedar, again very light weight timbers. I'm keeping them light by fairing with epoxy and 5 micron hollow glass spheres (balloons) as well. We also use the light weight sanding and compression strengh fillers when needed. These again are expensive.

Since we (Canberra Sea Scouts) are able to restore the boats fully, we are changing all hardware that's heavy. Swaping heavier Fico and Seacraft blocks for lighter Ronstans. Harken make the lightest equivalent bits. I'm also doing my own boat, she is 235 lbs(the minimum Class restriction), I hope my wife can afford Harken.

The lighter boats are our training boats. If a child stood in a beached boat they would put a footprint in the floor, at the least. This they have done in the past.

I did a Google search for, 'Epoxy foam sandwich composites'. The noticable factor on all sites was the increase in expense, due to the increase in time needed. Some sites quoted up to 300% increase in the time required to build over normal materials and factored-in the extra time taken as an expense. ...300%? :eek:

I now understand why modern racing dinghies are worth half a year's salery.

Warren.

ps, I think your new boat will look stunning, whichever way you choose to go.

[ 10-22-2002, 10:41 PM: Message edited by: Wild Wassa ]

[JimConlin]

Foam core composite, particularly Core-Cell, is hot among the over 20' go-fast set.
See http://www.atc-chem.com/
It's very light, at 5#/ft^3 for the most commonly used grade, has good sheer strength, is quite tough and doesn't rot. Not cheap. Close to $3/ft^2 for 1/2" 5# stuff. For small boats, the problem is compressive strength (about 115 psi). If you plan to visit a beach, drop a winch handle or thump a dock, it needs a heavier glass layup than you'd want on a small boat. For bigger boats, it works out very light. I'm now building a 28' trimaran in Core-Cell. The main hull, decks and cockpit are nearly done, and I can still lift one end out of the crade.
Save foam for the next project.

[john welsford]

The cost of labour is always an issue, we none of us have as much spare time as we would like and must use it wisely. There is an old saying , that you can get 90% of the result with 10% of the cost, and I think that glued lapstrake ply is in my experience a really pracical tradeoff in the weight v/s financial and other costs.
To cold mould the boat requires a very comprehensive mould over which to layup the skin, to make a really light cedar strip boat is both very labour intensive and requires really close mould stations ( more labour) and a lot of fibreglass, I have built a couple of foam/glass sandwich boats and would run screaming if I was asked to do another, especially in a very small light boat, but really enjoyed building Huffboat which is 16ft 8in long, 3 ft 8in wide and with built in bouyancy tanks, a really strong gunwale and substantial ply web frames weighs in well under 90 lbs. Labour content about 60 hours all up.( including making 30 wooden clothes pin and wedge type clamps with which to clamp the plank edges together.

JohnW
John


Originally posted by Bruce Taylor:
Or white cedar lapstrake, for that matter.

As some of you know, I've been designing an open camping dinghy. The boat (LOA 14', beam 4') will ride on a car from time to time, and might sometimes need to be launched from a bridge embankment or hoisted over a beaver dam. So, I'm building as light as I can (understanding, of course, that the craft will need to be treated gently, like an Adirondack guideboat or cedar canoe).

My question is: which of the common methods of construction yields the lightest boat?

Having built a cedar lapstrake canoe, I know these boats can be very light indeed (my Piccolo is a bit over 50 lbs, & Rushton's Sairy Gamp was just a bit heavier than a newborn baby: 10 1/2 pounds).

My first thought was to build an "ultralight" lap/ply boat. Tom Hill seems to enjoy carrying his boats on his fingertips, like a barmaid, and that certainly makes them look light. However, I'm told that his fingers are enormously strong from years of planing plywood against the grain, LOL.

And then there's glassed cedar-strip, as popularized by "Mac" McCarthy, Ted Moores, and others. That certainly seems to make for a lightweight boat.

So: which should I use?

For this project, I'm not really too interested in geodesic aerolite, carbon-fibre layups, or any other space-age miracle substances. I don't feel like making a skin boat, either.

Here's a rough model of the dinghy:

http://www.imagestation.com/picture/sraid36/pe8f62492203124b6f69979af1a06077a/fd256776.jpg

[NormMessinger]

Bruce, I was only about 10% serious when I suggested foam core. My choice in your case would be cedar strip but beings you asked: The Long EZ has less than 100 lbs of metal in it not counting the engine and only half dozen or so small pieces of wood where hard points or extra stiffness is required. The rest is one sort of foam or another covered with fiberglass and epoxy. The precess is called moldless construction where in the foam is cut to give form and the glass is applied to give strength.

I'm not sure that Core-Cell is the same stuff. Possibly though a heavier version. Their work methods are a little different as well.

Now, if you were to make your boat thus, from inside out: Paint, two plies 10 oz glass, 3/8" foam, 2 plies glass, one layer thin wood veneer, CPES, bright finish, why, even Cleek might think it a wooden boat.

Enough fo this foolishness. Carry on.

--Norm

[Bruce Taylor]

Hmm. 3 votes for lap, 3 for strip and 2 for cold moulding. I'd take this to the Supreme Court, but they're sure to rule in favour of GRP.

If it's a tossup, I guess it'll be lap ply...if only because it's quicker and less goopy. Thanks, guys.

[ion barnes]

I am with Norm on this one. Divinycell or Core-cell sheets glassed both sides. Without ribs or frames, I would build it lapstrake style. The heavier glass layup would be on the inside for resistance to wear and tear by crew but also any impact from the outside would cause an compression of the outter skin and expansion of the inner skin, hence the heavier inside skin. Might cost more than you are prepared for. Suggest looking up Bruce Nicholson"s book obout coldmolding.

[John Gearing]

Okay, let's talk some numbers here. According to Tom Hill, his "Barbara Jean" rowing skiff is about 15 feet by 38 inches and built with his method runs around 75 pounds. He has a sweet little sailing/rowing sharpie that is 12 feet by about 4 and this is projected at 90 lbs. To him, these are "ultralight" and "light" weights respectively. The questions now become a) how much are you capable of lifting onto your cartop, b) are there any mechanical aids to assist the human source, and c) how much can your cartop take. I'd guess the cartop must be able to take 100 lbs or more or we wouldn't even be having this discussion, so it must be the human factors aspect that sets the weight limitation. If it were me, I'd see if I couldn't add a leeboard and sailing rig to the "Barbara Jean" model, or else get Tom to gen me up a clipper version of the sharpie, but that's just my two cents. The real question is what is the max weight and what is a comfortable weight (leaving margin---i.e. not asking the human to supply max effort with every lift).

[George Roberts]

John Gearing has the right idea.

First, you decide how much weight you can handle. Then decide how strong the boat has to be. Then select among the options.

Cedar is a poor choice for cold molding. Foam is a poor choice for cored/glass construction. Both are subject to serious damage to light hulls under light concentrated loads.

Tom McNaughton suggests that wood core/glass faced construction is the lightest for equivalent strength among the hull types he builds.

A suitable cheap wood core/glass faced or plywood hull might weight under 11oz/sqft.

[Steve Lansdowne]

Having built a Whisp, which was designed as a lightweight, I'd caution you to give consideration to how you'll transport it. Light boats can be car topped, but the Sitka spruce outwales on my Whisp were easily damaged when I turned it over to put on the rollers of my roof rack while cartopping it. I ended up adding some oak corner molding over the wales, which increased the weight.

[imported_Conrad]

Double planked red cedar strip- 1/2" total, glass on the exterior bottom only. Varnished or painted only, no epoxy sealer. Builder's foam, 1", between 1/8" doorskins for seats and a few ribs. I once glued up, with epoxy, a "2X4" from door skins and 2lb/ft^3 2" builder's foam, pink, 4' long. It would support my 200lbs spanning two chairs and weighed nothing. The foam can be heat formed into ribs and topped with door skins or carbon fiber tape (1") for a very stiff, almost weightless structural member. Go Wild! Estimated total weight with rails, seats, etc. would be 1.75 lbs/foot^2

[ 10-23-2002, 08:29 PM: Message edited by: Conrad S. ]

[Bruce Taylor]

John G. -- You raise a good point -- I should probably just set a weight goal and shave down the scantlings until it's attained (or not).

On the other hand, it's interesting to hear opinions. It would be even better to see some hard data. Jim Conlin suggests I make some test planks and "beat them up," which is good advice...but I was kinda hoping somebody else had done the research.

I haven't heard of "Barbara Jean." It's not mentioned in Hill's book, is it?

[John Gearing]

Bruce--
It's not in the book. When I got my book there was a separate brochure inside featuring about ten of Tom Hill's designs, including Barbara Jean.

[imported_Paul VandenBosch]

Epoxy lapstrake, 4mm birch marine plywood. The laps give you a lot of stiffness to the hull. Use douglas fir framing only as you see necessary to reduce flexing. Maybe add a small plywood doubler on the floor to take wear and tear from your feet. Douglas fir inwhale and outwhale, each 1/2"x1-1/4", with a 3" 3/4"x1-1/4" spacer every foot between the inwhale and the plywood hull sheer. If the hull is stiff enough, eliminate thwarts and use a cane folding seat. You will probably need one or two thwarts, but if you are careful entering and exiting the boat you can probably do without. Use epoxy sparingly and only on the joints, not as a sealant. Use glass tape and epoxy along the bottom keel. Finish clear with two part polyurethane.

[brian.cunningham]

Glued ply
CLC's Annapolis Wherry (http://www.clcboats.com/wherry.php3?cart_id=CREATE) is
Length: 17' 9"
Beam: 38"
Weight: 65 lbs
http://www.clcboats.com/images/wherry2.jpg

Woodenboat sells, ultralight boat building.

You could also go skin-on-frame

[Meerkat]

My $.02:

Carvel, could moulding and strip plank are labor intensive. Lots of drudge work: caulking, fairing, stapling.

Lapstrake much less so. Glued lapstrake and S&G probably are the same in terms of time, but there's more drudge work in S&G compared to Glued lapstrake. I think you learn more skills with lapstrake compared to s&g - and end up with a prettier boat.

I've heard that traditional lapstrake can be less expensive then ply glued lapstrake, but my recent experience pricing wood makes me wonder about this. Downside of trad. lapstrake is that it likes to stay wet and isn't as structurally strong as glued lapstrake. Trad. seems like more work and is heavier given the need for all those frames.

[Meerkat]

Bruce, do you think that weight is achievable for an amateur?

BTW, that's "Mr. CLC" himself at the oars in the pic. I met him at the PT Woodenboat Festival.

[Bruce Taylor]

Thanks for the picture, Brian. I suppose it's just an effect of camera angle, but that boat doesn't look eighteen feet long.

Meerkat -- Trad. lapstrake can be very inexpensive, if you have access to decent local woods. The white cedar in my Piccolo cost me about fifty U.S. cents a boardfoot. The white ash, set me back about a dollar a b.f. A box of bronze ring nails is about ten bucks U.S., and clench nails are only a little more. I used a bit of googe for laminations and scarfing, but I certainly didn't buy it in gallon cans.

Is it necessarily heavier? I'm not so sure. If you compare Rushton's boats to equivalent hulls in other materials they fare pretty well. Ever hoisted an Adirondack guideboat?


Bruce, do you think that weight is achievable for an amateurI guess I'll find out!

[Keith Wilson]

How light you can go is really a function of how fragile a boat you are willing to put up with. Considering only mostly wood boats, I'd guess that for a given weight ply lapstrake (no glass) would be more rigid due to increased shell thickness and because of the double-thickness stringers formed by the laps. This is because wood is less dense than glass/epoxy, so an equivalent weight hull will be thicker. Cedar-core fiberglass would probably be more tolerant of abuse, and would handle more impact, abrasion, and deflection without damage, because of the greater tensile strength of the outer layers of the composite hull.

For ultimate light weight, you could use Kevlar/epoxy, perhaps over a foam or balsa core. This isn't so exotic any more, although it's not cheap; folks build themselves Kevlar canoes fairly regularly around here. Cutting the fabric requires carbide or ceramic tools, since it instantly dulls steel blades.

If it were my boat, I'd build it ply lapstrake. The amount of drudgery needed to fair up a strip-planked hull, both before and after glassing, does not appeal to me. Plywood comes flat and smooth, and is mostly self-fairing if the mold is accurate (and from looking at your model, yours definitely will be) Sanding and finishing labor is minimal if you use a heat gun and scraper to remove epoxy drips. Besides, it looks pretty, at least if one takes care to get the plank lines right.

Congratulations on the model, BTW. If that lovely thing is what you call "quick and dirty" I can't wait to see the boat.

[Sailman58]

Chalk up another vote for glued plywood lap. Put a layer of glass on the outside of the garboard to take wear and tear. If the boat is stiff enough, use a removable rowing seat to replace the thwart, it also keeps the interior of the boat more open. Maybe more glass or a short floorboard to take footwear. Removing the seat and floorboard will make it easier to get the boat up on the roof.

Ron

[brian.cunningham]

Originally posted by Bruce Taylor:
Thanks for the picture, Brian. I suppose it's just an effect of camera angle, but that boat doesn't look eighteen feet long. http://www.clcboats.com/images/wherryfixedseat.jpg
more pics on their site.

[ 10-24-2002, 03:10 PM: Message edited by: brian.cunningham ]

[JimConlin]

Curiously, the example of the Annapolis wherry shown at the WB show in Rockland measured significantly wider than 38". 42", if memory serves. Call me confused. The young fellow attending the stand had no explanation.

Regarding the earlier part of the thread, about the choice of planking material, the weight of 6mm okoume glued lapstrake, allowing 10% for laps is about .64 lbs/ft^2 and the common strip canoe scantlings (1/4" cedar, 6oz. glass) is about .78 lbs/ft^2. The strip composite, having its strength in the skins, gets the nod for stiffness.
I can't think of a finishing system for the okoume glued lapstrake which will produce the scuff resistance of the glass overlay. The full epoxy-LPU treatment, i'm sad to report, is quite fragile.
Which method takes more labor depends a lot on your methods. Strip is very fast if you have the jigs for milling strips, use an electric stapler and get the right routine for fairing and glassing. Lapstrake can be fast if you have already built the Hill-method mold or have lined-off and patterns for the planks.

Which you'd rather look at is an individual decision.
Which is more practical is an easier decision.

[Sailman58]

Jim,

If the beam was about 42" and the boat was 15 feet instead of 20, you were looking at the Chester Yawl prototype, not the Annapolis Wherry.

[JimConlin]

I didn't measure teh length. I recollect that it was labelled "annapolis wherry'. Why would CLC show a prototype instead of a 'production' design?

No big thing.

[Bruce Taylor]

the weight of 6mm okoume glued lapstrake, allowing 10% for laps is about .64 lbs/ft^2 and the common strip canoe scantlings (1/4" cedar, 6oz. glass) is about .78 lbs/ft^2.Thanks, Jim...just what I needed.

[Kermit]

Jack--Montfort's "guy wires" are kevlar, and they are only there because the fabric has durn little diagonal strength. With any wood at all, the kevlar would likely just be an expensive and time consuming redundancy. But I like the intuitive flash. Made me think!

Oh. My vote's glued lap ply.

[ 10-25-2002, 04:03 PM: Message edited by: Kermit ]

[Bruce Taylor]

Clinker ply, by a landslide.